The present invention relates generally to centrifugal pumps, and more particularly, to a method and apparatus of detecting low flow and/or cavitation in a centrifugal pump using voltage and current data acquired from voltage and current sensors in the pump motor controller assembly.
Submersible motor centrifugal pumps are commonly used for drinking water supply, irrigation, and de-watering as well as in offshore applications. Typically, both the motor as well as the pump are submerged and installed in deep wells down to 1,000 m. Moreover, motor power often reaches up to 2,000 kW and voltage up to 10,000 V. As a result of the remote location of these pumps, condition monitoring and detection of undesirable conditions at an early stage are often difficult.
Low flow and cavitation in centrifugal pumps may be caused by several conditions. For example, cavitation can result when air bubbles form in low pressure regions inside the pump. A phenomenon typically referenced as vaporization. As the bubbles collapse or degrade, they etch away at the impeller fins in the pump. Over time, this condition, often discernible as a xe2x80x9cpumping gravelxe2x80x9d sound coming from the pump, can lead to breakdown or failure of the pump. xe2x80x9cLow flowxe2x80x9d or restricted output of the pump occurs as a result of restriction due to obstructions at the input or output of the pump. Recirculation of fluid within the pump housing can result from such a restriction. The recirculation causes swirling vortices between blades of the impeller. This swirling effect reduces the torque in the pump and ultimately restricts the output of the pump clearly, an undesirable condition.
Because of the location of the submersible pump during operation, it is typically difficult to detect the onset of cavitation or low flow. A number of systems have been developed to detect cavitation or low flow but these systems require additional instrumentation that is integrated with the pump. The additional instrumentation increases the complexity of the pump as well as the costs of manufacturing.
It would therefore be desirable to design a pump assembly wherein low flow and/or cavitation are quickly identified and detected without additional instrumentation in the pump.
The present invention is directed to a centrifugal pump wherein voltage and current data are detected from voltage and current sensors in the pump motor controller to determine low flow and/or cavitation in the pump overcoming the aforementioned drawbacks. A power signal is then generated from the voltage and current data and spectrally analyzed to determine the low flow or cavitation in the pump. As such, unwanted operational conditions resulting from improper pump operation may be detected and a warning or maintenance flag provided without additional transducers and other instruments on the motor or pump.
Accordingly, motor power is used to determine the presence of a low flow and/or cavitation condition in the pump, i.e. a torque loss and vaporization. Power is preferably determined from voltage and current data acquired from a three-phase motor. At initial setup of the pump assembly, a baseline signal is determined from the pump known to be operating in a normal, healthy condition. The baseline signal or data is then used for comparison with equivalent signals derived from operational data so that deviations from normal, healthy operation can be readily identified.
Voltage and current data are collected for a relatively short period of time such as one second and a corresponding power signal is then generated. The power signal is then analyzed with a fast Fourier transform (FFT) or band-pass filtered to locate increased noise levels in the power signal, relative to a baseline signal, within a predetermined frequency range. By comparing the energy, within the frequency range of interest, of the transformed or filtered signal with the baseline signal, increased noise levels indicating the presence of cavitation or low flow conditions can be readily identified. The relative increase in noise level is an indication of the magnitude of the low flow or cavitation. Preferably, a maintenance warning or flag is then provided to an operator or other technician so that, if needed, the pump may be shut down or the operating conditions adjusted to eliminate the cause of the fault condition.
Therefore, in accordance with one aspect of the present invention, a motor control for a motor-driven pump is provided. The controller includes at least one voltage sensor and at least one current sensor and is configured to receive a voltage and a current signal of the pump in operation from the at least one voltage sensor and at least one current sensor. The controller is further configured to determine a power signal from the voltage signal, and the current signal, to generate a real-time spectrum analysis of the power signal and to extract critical features from the spectrum. The controller is also configured to determine at least one of a low flow and a cavitation condition in the pump from the spectrum analysis.
In accordance with another aspect of the present invention, a computer readable storage medium having stored thereon a computer program to detect and signal improper operation of a motor-driven pump is provided. The computer program represents a set of instructions that when executed by a processor causes the processor to determine a real-time power signal of power in a pump motor assembly. The set of instructions further causes the processor to perform a spectrum analysis on the real-time power signal over several time cycles and generate a real-time frequency spectrum therefrom. The computer program then compares real-time data with baseline operation of a system known to be in good operating condition and operating at nominal flow conditions. The computer program then determines differences between the baseline data and the data derived from current operating conditions. If the differences exceed a threshold, the computer program provides an external notification signaling improper operation in the pump.
In accordance with yet a further aspect of the present invention, a method of monitoring proper operation in a centrifugal pump motor includes the step of generating a baseline frequency spectrum of a pump known to be operating properly. The method further includes the step of generating a real-time frequency spectrum of the pump in operation from a power signal derived from voltage and current signals acquired from sensors in a motor of the pump, then extracting from this spectrum the energy contained in a prescribed frequency range. This energy is then compared to that obtained from the baseline frequency spectrum and an undesirable pump operating condition is determined therefrom. If an undesirable condition exists, a signal indicating the presence of the condition is provided.